Insulin pumps are widely available and are used by diabetic people to automatically deliver insulin over extended periods of time. All currently available insulin pumps employ a common pumping technology, the syringe pump. In a syringe pump, the plunger of the syringe is advanced by a lead screw that is turned by a precision stepper motor. As the plunger advances, fluid is forced out of the syringe, through a catheter to the patient. The choice of the syringe pump as a pumping technology for insulin pumps is motivated by its ability to precisely deliver the relatively small volume of insulin required by a typical diabetic (about 0.1 to about 1.0 cm3 per day) in a nearly continuous manner. The delivery rate of a syringe pump can also be readily adjusted through a large range to accommodate changing insulin requirements of an individual (e.g., basal rates and bolus doses) by adjusting the stepping rate of the motor. While the syringe pump is unparalleled in its ability to precisely deliver a liquid over a wide range of flow rates and in a nearly continuous manner, such performance comes at a cost. Currently available insulin pumps are complicated and expensive pieces of equipment costing thousands of dollars. This high cost is due primarily to the complexity of the stepper motor and lead screw mechanism. These components also contribute significantly to the overall size and weight of the insulin pump. Additionally, because of their cost, currently available insulin pumps have an intended period of use of up to two years, which necessitates routine maintenance of the device such as recharging the power supply and refilling with insulin.
U.S. Pat. No. 6,375,638 of Clyde Nason and William H. Stutz, Jr., entitled “Incremental Motion Pump Mechanisms Powered by Shape Memory Alloy Wire or the Like,” issued Apr. 23, 2002, and naming Medtronic MiniMed, Inc. as the assignee, which patent is incorporated herein in its entirety by this reference, describes various ratchet type mechanisms for incrementally advancing the plunger of a syringe pump. The ratchet mechanisms are actuated by a shape memory alloy wire. The embodiments taught by Nason et al. involve a large number of moving parts, and are mechanically complex, which increases size, weight and cost, and can reduce reliability.